The present invention relates to a duplex field bus system and particularly relates to a duplex field bus system for easily maintaining high reliability in a multi-drop connection structure in which duplex transmission lines are constructed and a plurality of field devices are connected to each transmission line.
The so-called "field devices", in general, detect physical quantities such as pressure, temperature, flow rate and the like at respective points in various kinds of plants, and convert the values of the detected physical quantities into electric signals and transmit the electric signals to upper-rank apparatuses through transmission lines. On the other hand the devices also receive control signals transmitted from the upper-rank apparatuses, controlling valves or the like in plants. The transmission of electric signals such as measurement signals, control signals and the like in transmission lines are standardized in the case where the signals are analog signals. In this case, analog current signals of 4 to 20 mA are transmitted between the field devices and the upper-rank apparatuses. In the transmission lines between the field devices and the upper-rank apparatuses, heretofore, one-directional communication is generally made by analog signals.
In recent years, a field device containing a micro-processor has been developed with the advance of the semi-conductor integrated circuit technique and has been put into practical use. According to the field device, not only one-directional communication can be made by analog signals through a transmission line but two-way communication can be made by digital signals through a transmission line. Further, the field device has been developed so that the range setting of the field device, the self diagnosis thereof, and the like, can be instructed remotely by using the two-way communication. The devices as described above have been disclosed in JP-A-58-48198 and JP-A-59-201535.
A typical example of the conventional transmission system will be described specifically hereunder with reference to FIG. 6. FIG. 6 shows an example of the structure of analog current output type field devices which require an external power supply. Field devices 101a, 101b and 101c are operated by electric power supplied from an external power supply 103 through a transmission line 102, and generate analog current signals as constant-current sources for passing currents corresponding to the respectively detected physical quantities through the transmission line 102. An upper-rank reception apparatus 104 receives the analog current signals (hereinafter referred to as "analog signals") flowing in a resistor (not shown) connected in series with the transmission line 102, by detecting the signals as voltages across the opposite ends of the resistor, and uses the signals as instruction values given to the field devices 101a, 101b and 101c. An upper-rank communication device 105 is connected to a desired point between the field devices 101a, 101b and 101c and the reception apparatus 104 and the external power supply 103. The upper-rank communication device 105 communicates with the field devices by using digital signals through the two-way communication.
As for the signal transmission method in the transmission line, there is known a method in which digital signals are superimposed on analog signals to perform communication by digital signals so as not to influence the analog signals, another method in which analog signals and digital signals are switched with each other to transmit the analog signals or digital signals, and yet another method in which signals are transmitted by using digital signals without the use of analog signals, and the like.
Recently, there has been proposed a field bus system in which a plurality of field devices are connected onto one transmission line by means of a multidrop system to perform two-way communication with digital signals. A typical example of the structure of the field bus system will be described hereunder with reference to FIG. 7. FIG. 7 shows an example of the structure of a system in which a plurality of field devices are tree-like connected to upper-rank apparatuses through a transmission line. Field devices 111a, 111b and 111c are operated through electric power supplied from an external electric source 113 through a transmission line 112. The field devices make two-way communication with an upper-rank reception apparatus 114 with digital signals through the transmission line 112 successively, so that processes such as transmission of detected physical quantities, reception of control values, and the like, are made. An upper-rank communication device 115 is connected between the field devices 111a, 111b and 111c and the reception apparatus 114 and the external power supply 113 and makes two-way communication with the field devices with digital signals. Terminators 116 are constituted by a resistor and a capacitor connected in series with each other, and are connected to opposite ends of the transmission line 112.
In the case where the system is shifted from the conventional system in FIG. 6 to the field bus system in FIG. 7, the upper-rank devices and the field devices must be replaced by devices adapted for the field bus system. Because the transmission line 102, however, can be directly used as the transmission line 112, the system can be shifted easily. In the field bus system, the number of field devices connected to the transmission line 112 can be increased. Accordingly, the field bus system has an advantage in that the system can be extended easily.
In the aforementioned prior art technique, however, not only is the number of field devices connected to one transmission line increased when the system is shifted from the conventional system to the field bus system, but communication is made by digital signals as well. In this respect, the field bus system has no consideration in the point of view of reliability against noise in comparison with the conventional system. Further, the field bus system has a problem in that the field devices, which have been controlled periodically, cannot be controlled when communication error occurs frequently.
In particular, in the case of failure in the transmission line, electric power cannot be supplied to the field devices connected to the transmission line at all. There is no consideration given to the fact of that all the field devices cannot be operated.
Although a method of providing the field bus as a duplex field bus in total may be considered as a technique for improving reliability of the transmission line, there arises a problem in that there is no merit for shifting the system from the conventional system in the point of view of cost performance because all the devices are required for each transmission line.